JDK动态代理步骤详解(源码分析)

动态代理步骤

1.创建一个实现接口InvocationHandler的类，它必须实现invoke方法

2.创建被代理的类以及接口

3.通过Proxy的静态方法

通过Proxy的静态方法

ProxyObject proxyObject = new ProxyObject();
    InvocationHandler invocationHandler = new DynamicProxy(proxyObject);
    ClassLoader classLoader = proxyObject.getClass().getClassLoader();
    ProxyObjectInterface proxy = (IRoom) Proxy.newProxyInstance(classLoader,new Class[]
    {ProxyObjectInterface.class},invocationHandler);
    proxy.execute();

    public class DynamicProxy implements InvocationHandler {
    private Object object;

    public DynamicProxy(Object object){
        this.object = object;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        Object result = method.invoke(object,args);
        return result;
    }
}

创建一个代理 newProxyInstance

public static Object newProxyInstance(ClassLoader loader,
                                          Class[] interfaces,
                                          InvocationHandler h)
        throws IllegalArgumentException
    {
        //检验h不为空，h为空抛异常
        Objects.requireNonNull(h);
        //接口的类对象拷贝一份
        final Class[] intfs = interfaces.clone();
        //进行一些安全性检查
        final SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
        }
        /*
         * Look up or generate the designated proxy class.
         *  查询（在缓存中已经有）或生成指定的代理类的class对象。
         */
        Class cl = getProxyClass0(loader, intfs);
        /*
         * Invoke its constructor with the designated invocation handler.
         */
        try {
            if (sm != null) {
                checkNewProxyPermission(Reflection.getCallerClass(), cl);
            }
            //得到代理类对象的构造函数，这个构造函数的参数由constructorParams指定
            //参数constructorParames为常量值：
            private static final Class[] constructorParams = { InvocationHandler.class };
            final Constructor cons = cl.getConstructor(constructorParams);
            final InvocationHandler ih = h;
            if (!Modifier.isPublic(cl.getModifiers())) {
                AccessController.doPrivileged(new PrivilegedAction() {
                    public Void run() {
                        cons.setAccessible(true);
                        return null;
                    }
                });
            }
            //这里生成代理对象，传入的参数new Object[]{h}后面讲
            return cons.newInstance(new Object[]{h});
        } catch (IllegalAccessException|InstantiationException e) {
            throw new InternalError(e.toString(), e);
        } catch (InvocationTargetException e) {
            Throwable t = e.getCause();
            if (t instanceof RuntimeException) {
                throw (RuntimeException) t;
            } else {
                throw new InternalError(t.toString(), t);
            }
        } catch (NoSuchMethodException e) {
            throw new InternalError(e.toString(), e);
        }
    }

先对h进行判空处理。
这段代码核心就是通过getProxyClass0(loader, intfs)得到代理类的Class对象，然后通过Class对象得到构造方法，进而创建代理对象。下一步看getProxyClass0这个方法。从1可知，先接口得到接口类，当接口的数量超过65535，则报异常。

//此方法也是Proxy类下的方法
    private static Class getProxyClass0(ClassLoader loader,
                                           Class... interfaces) {
        if (interfaces.length > 65535) {
            throw new IllegalArgumentException("interface limit exceeded");
        }

        // If the proxy class defined by the given loader implementing
        // the given interfaces exists, this will simply return the cached copy;
        // otherwise, it will create the proxy class via the ProxyClassFactory
        //意思是：如果代理类被指定的类加载器loader定义了，并实现了给定的接口interfaces，
        //那么就返回缓存的代理类对象，否则使用ProxyClassFactory创建代理类。
        return proxyClassCache.get(loader, interfaces);
    }

proxyClassCache 是一个弱引用的缓存
这里看到proxyClassCache，有Cache便知道是缓存的意思，正好呼应了前面Look up or generate the designated proxy class。查询（在缓存中已经有）或生成指定的代理类的class对象这段注释。

在进入get方法之前，我们看下 proxyClassCache是什么？高能预警，前方代码看起来可能有乱，但我们只需要关注重点即可。

private static final WeakCache[], Class>
        proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
//K代表key的类型，P代表参数的类型，V代表value的类型。
// WeakCache[], Class>  proxyClassCache  说明proxyClassCache存的值是Class对象，正是我们需要的代理类对象。
final class WeakCache {
    private final ReferenceQueue refQueue
        = new ReferenceQueue<>();
    // the key type is Object for supporting null key
    private final ConcurrentMap>> map
        = new ConcurrentHashMap<>();
    private final ConcurrentMap, Boolean> reverseMap
        = new ConcurrentHashMap<>();
    private final BiFunction subKeyFactory;
    private final BiFunction valueFactory;
    public WeakCache(BiFunction subKeyFactory,
                     BiFunction valueFactory) {
        this.subKeyFactory = Objects.requireNonNull(subKeyFactory);
        this.valueFactory = Objects.requireNonNull(valueFactory);
    }

其中map变量是实现缓存的核心变量，他是一个双重的Map结构: (key, sub-key) -> value。其中key是传进来的Classloader进行包装后的对象，sub-key是由WeakCache构造函数传人的KeyFactory()生成的。value就是产生代理类的对象，是由WeakCache构造函数传人的ProxyClassFactory()生成的。如下，回顾一下:

proxyClassCache是个WeakCache类的对象，调用proxyClassCache.get(loader, interfaces)； 可以得到缓存的代理类或创建代理类（没有缓存的情况）。

说明WeakCache中有get这个方法。先看下WeakCache类的定义（这里先只给出变量的定义和构造函数），继续看它的get（）；

//K和P就是WeakCache定义中的泛型，key是类加载器，parameter是接口类数组
public V get(K key, P parameter) {
        //检查parameter不为空
        Objects.requireNonNull(parameter);
         //清除无效的缓存
        expungeStaleEntries();
        // cacheKey就是(key, sub-key) -> value里的一级key，
        Object cacheKey = CacheKey.valueOf(key, refQueue);
        // lazily install the 2nd level valuesMap for the particular cacheKey
        //根据一级key得到 ConcurrentMap>对象。如果之前不存在，则新建一个ConcurrentMap>和cacheKey（一级key）一起放到map中。
        ConcurrentMap> valuesMap = map.get(cacheKey);
        if (valuesMap == null) {
            ConcurrentMap> oldValuesMap
                = map.putIfAbsent(cacheKey,
                                  valuesMap = new ConcurrentHashMap<>());
            if (oldValuesMap != null) {
                valuesMap = oldValuesMap;
            }
        }

        // create subKey and retrieve the possible Supplier stored by that
        // subKey from valuesMap
        //这部分就是调用生成sub-key的代码，上面我们已经看过怎么生成的了
        Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
        //通过sub-key得到supplier
        Supplier supplier = valuesMap.get(subKey);
        //supplier实际上就是这个factory
        Factory factory = null;

        while (true) {
            //如果缓存里有supplier ，那就直接通过get方法，得到代理类对象，返回，就结束了，一会儿分析get方法。
            if (supplier != null) {
                // supplier might be a Factory or a CacheValue instance
                V value = supplier.get();
                if (value != null) {
                    return value;
                }
            }
            // else no supplier in cache
            // or a supplier that returned null (could be a cleared CacheValue
            // or a Factory that wasn't successful in installing the CacheValue)
            // lazily construct a Factory
            //下面的所有代码目的就是：如果缓存中没有supplier，则创建一个Factory对象，把factory对象在多线程的环境下安全的赋给supplier。
            //因为是在while（true）中，赋值成功后又回到上面去调get方法，返回才结束。
            if (factory == null) {
                factory = new Factory(key, parameter, subKey, valuesMap);
            }

            if (supplier == null) {
                supplier = valuesMap.putIfAbsent(subKey, factory);
                if (supplier == null) {
                    // successfully installed Factory
                    supplier = factory;
                }
                // else retry with winning supplier
            } else {
                if (valuesMap.replace(subKey, supplier, factory)) {
                    // successfully replaced
                    // cleared CacheEntry / unsuccessful Factory
                    // with our Factory
                    supplier = factory;
                } else {
                    // retry with current supplier
                    supplier = valuesMap.get(subKey);
                }
            }
        }
    }

所以接下来我们看Factory类中的get方法。接下来看supplier的get()

public synchronized V get() { // serialize access
            // re-check
            Supplier supplier = valuesMap.get(subKey);
            //重新检查得到的supplier是不是当前对象
            if (supplier != this) {
                // something changed while we were waiting:
                // might be that we were replaced by a CacheValue
                // or were removed because of failure ->
                // return null to signal WeakCache.get() to retry
                // the loop
                return null;
            }
            // else still us (supplier == this)
            // create new value
            V value = null;
            try {
                 //代理类就是在这个位置调用valueFactory生成的
                 //valueFactory就是我们传入的 new ProxyClassFactory()
                //一会我们分析ProxyClassFactory()的apply方法
                value = Objects.requireNonNull(valueFactory.apply(key, parameter));
            } finally {
                if (value == null) { // remove us on failure
                    valuesMap.remove(subKey, this);
                }
            }
            // the only path to reach here is with non-null value
            assert value != null;

            // wrap value with CacheValue (WeakReference)
            //把value包装成弱引用
            CacheValue cacheValue = new CacheValue<>(value);

            // put into reverseMap
            // reverseMap是用来实现缓存的有效性
            reverseMap.put(cacheValue, Boolean.TRUE);

            // try replacing us with CacheValue (this should always succeed)
            if (!valuesMap.replace(subKey, this, cacheValue)) {
                throw new AssertionError("Should not reach here");
            }

            // successfully replaced us with new CacheValue -> return the value
            // wrapped by it
            return value;
        }
    }

拨云见日，来到ProxyClassFactory的apply方法，代理类就是在这里生成的。

首先看proxyClassCache的定义WeakCache[], Class>，泛型里面第一个表示加载器K，第二个表示接口类P，第三个则是生成的代理类V。而V的生成则是通过ProxyClassFactory生成的。调用其apply（）；

//这里的BiFunction是个函数式接口，可以理解为用T，U两种类型做参数，得到R类型的返回值
private static final class ProxyClassFactory
        implements BiFunction[], Class>
    {
        // prefix for all proxy class names
        //所有代理类名字的前缀
        private static final String proxyClassNamePrefix = "$Proxy";
        // next number to use for generation of unique proxy class names
        //用于生成代理类名字的计数器
        private static final AtomicLong nextUniqueNumber = new AtomicLong();
        @Override
        public Class apply(ClassLoader loader, Class[] interfaces) {
            Map, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
            //验证代理接口，可不看
            for (Class intf : interfaces) {
                /*
                 * Verify that the class loader resolves the name of this
                 * interface to the same Class object.
                 */
                Class interfaceClass = null;
                try {
                    interfaceClass = Class.forName(intf.getName(), false, loader);
                } catch (ClassNotFoundException e) {
                }
                if (interfaceClass != intf) {
                    throw new IllegalArgumentException(
                        intf + " is not visible from class loader");
                }
                /*
                 * Verify that the Class object actually represents an
                 * interface.
                 */
                if (!interfaceClass.isInterface()) {
                    throw new IllegalArgumentException(
                        interfaceClass.getName() + " is not an interface");
                }
                /*
                 * Verify that this interface is not a duplicate.
                 */
                if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {
                    throw new IllegalArgumentException(
                        "repeated interface: " + interfaceClass.getName());
                }
            }
            //生成的代理类的包名 
            String proxyPkg = null;     // package to define proxy class in
            //代理类访问控制符: public ,final
            int accessFlags = Modifier.PUBLIC | Modifier.FINAL;
            /*
             * Record the package of a non-public proxy interface so that the
             * proxy class will be defined in the same package.  Verify that
             * all non-public proxy interfaces are in the same package.
             */
            //验证所有非公共的接口在同一个包内；公共的就无需处理
            //生成包名和类名的逻辑，包名默认是com.sun.proxy，
            // 类名默认是$Proxy 加上一个自增的整数值
            //如果被代理类是 non-public proxy interface ，则用和被代理类接口一样的包名
            for (Class intf : interfaces) {
                int flags = intf.getModifiers();
                if (!Modifier.isPublic(flags)) {
                    accessFlags = Modifier.FINAL;
                    String name = intf.getName();
                    int n = name.lastIndexOf('.');
                    String pkg = ((n == -1) ? "" : name.substring(0, n + 1));
                    if (proxyPkg == null) {
                        proxyPkg = pkg;
                    } else if (!pkg.equals(proxyPkg)) {
                        throw new IllegalArgumentException(
                            "non-public interfaces from different packages");
                    }
                }
            }
            if (proxyPkg == null) {
                // if no non-public proxy interfaces, use com.sun.proxy package
                proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
            }

            /*
             * Choose a name for the proxy class to generate.
             */
            long num = nextUniqueNumber.getAndIncrement();
            //代理类的完全限定名，如com.sun.proxy.$Proxy0.calss
            String proxyName = proxyPkg + proxyClassNamePrefix + num;

            /*
             * Generate the specified proxy class.
             */
            //核心部分，生成代理类的字节码
            byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
                proxyName, interfaces, accessFlags);
            try {
                //把代理类加载到JVM中，至此动态代理过程基本结束了
                return defineClass0(loader, proxyName,
                                    proxyClassFile, 0, proxyClassFile.length);
            } catch (ClassFormatError e) {
                /*
                 * A ClassFormatError here means that (barring bugs in the
                 * proxy class generation code) there was some other
                 * invalid aspect of the arguments supplied to the proxy
                 * class creation (such as virtual machine limitations
                 * exceeded).
                 */
                throw new IllegalArgumentException(e.toString());
            }
        }
    }

然后调用getMethod（）,将equals(),hashcode(),toString()等方法添加进去。然后遍历所有接口的方法，添加到代理类中。最后将这些方法进行排序。

private static List getMethods(Class[] interfaces) {
        List result = new ArrayList();
        try {
            result.add(Object.class.getMethod("equals", Object.class));
            result.add(Object.class.getMethod("hashCode", EmptyArray.CLASS));
            result.add(Object.class.getMethod("toString", EmptyArray.CLASS));
        } catch (NoSuchMethodException e) {
            throw new AssertionError();
        }

        getMethodsRecursive(interfaces, result);
        return result;
    }
private static void getMethodsRecursive(Class[] interfaces, List methods) {
        for (Class i : interfaces) {
            getMethodsRecursive(i.getInterfaces(), methods);
            Collections.addAll(methods, i.getDeclaredMethods());
        }
    }

最后输出相关proxy class

package com.zhb.jdk.proxy;
import java.io.FileOutputStream;
import java.io.IOException;
import java.lang.reflect.Proxy;

import com.zhb.jdk.dynamicProxy.HelloworldImpl;

import sun.misc.ProxyGenerator;

/**
 * @author ZHB
 * @date 2018年8月31日下午11:35:07
 * @todo TODO
 */
public class DynamicProxyTest {

    public static void main(String[] args) {

        IUserService target = new UserServiceImpl();
        MyInvocationHandler handler = new MyInvocationHandler(target);
        //第一个参数是指定代理类的类加载器（我们传入当前测试类的类加载器）
        //第二个参数是代理类需要实现的接口（我们传入被代理类实现的接口，这样生成的代理类和被代理类就实现了相同的接口）
        //第三个参数是invocation handler，用来处理方法的调用。这里传入我们自己实现的handler
        IUserService proxyObject = (IUserService) Proxy.newProxyInstance(DynamicProxyTest.class.getClassLoader(),
                target.getClass().getInterfaces(), handler);
        proxyObject.add("陈粒");

        String path = "D:/$Proxy0.class";
        byte[] classFile = ProxyGenerator.generateProxyClass("$Proxy0", HelloworldImpl.class.getInterfaces());
        FileOutputStream out = null;

        try {
            out = new FileOutputStream(path);
            out.write(classFile);
            out.flush();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                out.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

    }
}
// Decompiled by Jad v1.5.8e2. Copyright 2001 Pavel Kouznetsov.
// Jad home page: http://kpdus.tripod.com/jad.html
// Decompiler options: packimports(3) fieldsfirst ansi space

import com.zhb.jdk.proxy.IUserService;
import java.lang.reflect.*;

public final class $Proxy0 extends Proxy
    implements IUserService
{

    private static Method m1;
    private static Method m2;
    private static Method m3;
    private static Method m0;
    //代理类的构造函数，其参数正是是InvocationHandler实例，
    //Proxy.newInstance方法就是通过通过这个构造函数来创建代理实例的
    public $Proxy0(InvocationHandler invocationhandler)
    {
        super(invocationhandler);
    }
     // Object类中的三个方法，equals，toString， hashCode
    public final boolean equals(Object obj)
    {
        try
        {
            return ((Boolean)super.h.invoke(this, m1, new Object[] {
                obj
            })).booleanValue();
        }
        catch (Error ) { }
        catch (Throwable throwable)
        {
            throw new UndeclaredThrowableException(throwable);
        }
    }

    public final String toString()
    {
        try
        {
            return (String)super.h.invoke(this, m2, null);
        }
        catch (Error ) { }
        catch (Throwable throwable)
        {
            throw new UndeclaredThrowableException(throwable);
        }
    }
    //接口代理方法
    public final void add(String s)
    {
        try
        {
            // invocation handler的 invoke方法在这里被调用
            super.h.invoke(this, m3, new Object[] {
                s
            });
            return;
        }
        catch (Error ) { }
        catch (Throwable throwable)
        {
            throw new UndeclaredThrowableException(throwable);
        }
    }

    public final int hashCode()
    {
        try
        {
            // 在这里调用了invoke方法。
            return ((Integer)super.h.invoke(this, m0, null)).intValue();
        }
        catch (Error ) { }
        catch (Throwable throwable)
        {
            throw new UndeclaredThrowableException(throwable);
        }
    }

    // 静态代码块对变量进行一些初始化工作
    static 
    {
        try
        {
            m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[] {
                Class.forName("java.lang.Object")
            });
            m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
            m3 = Class.forName("com.zhb.jdk.proxy.IUserService").getMethod("add", new Class[] {
                Class.forName("java.lang.String")
            });
            m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
        }
        catch (NoSuchMethodException nosuchmethodexception)
        {
            throw new NoSuchMethodError(nosuchmethodexception.getMessage());
        }
        catch (ClassNotFoundException classnotfoundexception)
        {
            throw new NoClassDefFoundError(classnotfoundexception.getMessage());
        }
    }
}

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